During surgical procedures such as cardiopulmonary bypass operations it is possible to replace the function of the lungs with an extracorporeal blood oxygenator. Venous blood is passed in an extracorporeal circuit through the oxygenator, arterialized (i.e. oxygenated) and returned to the patient's body. One well known type of blood oxygenator is the bubble oxygenator in which treating gas (e.g. oxygen) bubbles are created and brought into direct contact with the venous blood stream, with resulting mass transfer of oxygen from the bubbles to the blood and of carbon dioxide from the blood to the bubbles. Although bubble blood oxygenators have been employed for many years with considerable success, their use is characterized by an inherent disadvantage, which is the need to create a blood foam to accomplish gas transfer. The bursting of gas bubbles into the blood stream during the creation of the foam may give rise to deleterious hemolysis. Furthermore, the blood foam must be thoroughly treated after oxygenation, typically in a defoamer and an arterial filter, to remove gaseous bubbles and emboli before the arterialized blood can be returned to the patient.
Another well known type of blood oxygenator, the membrane oxygenator, does not require formation of a blood foam to accomplish gas transfer. Blood and treating gas streams flow in the oxygenator separated by a semipermeable membrane, through which oxygen passes to the blood, carbon dioxide passes from the blood, while water and other non-gaseous blood components do not pass. In designing a membrane oxygenator for use in typical extracorporeal circuits, it is a desired goal to provide adequate gas transfer capacity to process the anticipated blood flows (from about 2 liters/min. to about 6 liters/min.) while maintaining a compact size for convenient handling, priming and operation, avoiding excessive blood and treating gas pressure drops and preventing significant trauma to blood components.
Blood membrane oxygenators of the general type having a pleated, semipermeable membrane held in a housing with support screens positioned in alternating blood and oxygen passageways are known (see, for example, U.S. Pat. Nos. 3,560,340; 3,612,281; 3,780,870 and 4,199,457). However, none of the membrane oxygenators disclosed in said U.S patents fully satisfy all of the criteria set forth above in the preceding paragraph. U.S. Pat. No. 4,028,252 discloses a blood dialyzer containing a single sheet of membrane and a single length of non-woven support material accordion-folded together to form alternating blood and dialysate flow paths separated by the membrane, with the support material being capable of interdigitation when folded upon itself. Accordingly, the support material is located on only one side of the membrane in this prior art dialyzer. U.S. Pat. No. 213,858 discloses the use of three-layer non-woven "supporting nets" in blood dialyzers and oxygenators, adjacent units of which are capable of interdigitation. However, even when such interdigitation occurs, the flow passageway defined by two portions of membrane separated by one of the supporting nets is at least about as thick as the middle layer of the supporting net and thus occupies more volume than desired in an extracorporeal blood oxygenator.